Fixes to respond_to? and warning tests for Ruby 2.0.0-preview1

In Ruby 2.0, respond_to? no longer returns true for protected methods unless you explicitly pass `true` as a second argument. This causes try_send to fail for @content blocks, keeping sass from knowing that a mixin supports blocks.

Also, Kernel.warn behaves slightly differently. Since the difference is in number of newlines, I figure it's a cosmetic change and made the most expedient fix I could think of.

Let me know if you see any issues!